1. Field of the Invention
The present invention relates to an amplifier for amplifying an input signal such as a video signal and outputting the amplified signal.
2. Description of the Related Art
Conventionally, a circuit as shown in FIG. 8 has been used as a processor circuit for a video signal. In this circuit, a pedestal level or a synch-tip level of a video signal is clamped by a clamp circuit 10 and the video signal is amplified by a pre-amplifier 12 and a main amplifier 14. An output of the main amplifier 14 is output to a co-axial power supply line 16 via a DC-cut capacitor Cdc.
As described, this circuit requires a capacitor Cdc for cutting the DC component. In the case of a video signal, because an input/output impedance is 75 Ω and a lower frequency component of the signal is approximately 60 Hz, in order to output the signal of the lower frequency component while preventing generation of a level shift and sag, the capacitance of the capacitor Cdc must be set to a very large value such as, for example, 470 μF-1000 μF. A capacitor having such a high capacitance is expensive and, moreover, requires a large space for provision of the capacitor and related wiring.
It is very important to reduce the cost and size in portable devices such as a digital video camera, and therefore, the capacitance of the capacitor is set to a minimum possible value. However, when the capacitance is reduced, the direct current component of the output changes, which may cause problems such as, for example, detection of a synchronizing signal for an output signal being difficult.
In consideration of this, another circuit is known in which an output of a capacitor Cdc is fed back to a main amplifier 14 via another capacitor Ca, as shown in FIG. 9. In this circuit, the capacitance of the capacitor Cdc can be set to approximately 22 μF-470 μF and the capacitance of the capacitor Ca can be set to approximately 10 μF to 22 μF. The output from the main amplifier 14 in this circuit is as shown in FIG. 10(b) for an input signal of a rectangular waveform as shown in FIG. 10(a) and an output through the capacitor Cdc is as shown in FIG. 10(c), which allows correction to a shape with the sag removed. By using such a sag correction circuit, it is possible to reduce the capacitance of the capacitor Cdc.
Even with the circuit shown in FIG. 9, however, a large capacitance is necessary for preventing a direct current shift. In addition, because of the recent trend of reduction in the voltage used as the power supply voltage, it is now more difficult to correct sag. That is, when a dynamic range in the output of the main amplifier 14 is not sufficient, the sharp extruding portion of the edges in FIG. 10 (b) is cut, and as a result, sufficient sag correction cannot be applied. Because of this, there is a problem in the video signal in that the synchronizing signal is suppressed and the synchronizing signal cannot be detected.